The proposed research is designed to achieve a useful understanding of the factors determining formation and maturation of germ cells. It includes an analysis of events of germ cell differentiation and a determination of the origin, composition and fate of unique cytoplasmic components present during this process. These components are discrete aggregates of dense fibrous material which have been observed in germ cells in both ovaries and testes in a wide range of species. They are frequently associated with clusters of mitochondria and occasionally with nuclear pores. The origin and fate of this material will be studied in tadpole oocytes and in the germ cells of male and female rats with electron microscopic, radioautographic and cell fractionation techniques. An additional aim of the present study is to distinguish the events of germ cell formation controlled by the cells themselves from the events determined by factors in their environment. Seminiferous tubules of the rat will be grown in culture where the effects of hormones, the influence of Sertoli cells and the roles of various nutritional and physical conditions may be examined. Parallel studies on seminiferous cells in immature rats will allow the comparison of events of germ cell differentiation under these two conditions.